whining engine noise

My 2007 Dodge Grand Caravan SXT has started to whine after running 100-150 miles at the time I slow down. After resting for a while there is no problem and then after 1oo-150 milesthe engine starts to whine again?? This first happened a couple of weeks ago when I filled up with 15% etrhanol. I then refilled with regular no lead and the sound went away until yesterday and the whine has started again???